Recovering precious metals from electronic waste

The electronic industry is a major consumer of precious metals. It’s estimated that 320 tons of gold are used annually in the manufacture of electronic products; this amount represents approximately 10 percent of gold produced worldwide each year. According to the Electronics Take Back Coalition, in 2010 27 percent of e-waste was recycled, and the remainder was dumped.

Apart from the environmental problems associated with dumping mountains of e-waste, there is significant economic potential for increased electronic gold recovery.

Precious Metals in Electronics

The American Journal of Engineering Research identified numerous metals used in the manufacture of electronics. These include relatively large quantities of copper, lead, tin, iron and aluminum, smaller amounts of mercury and cadmium, and trace amounts of other metals including gold, silver and palladium. From a value perspective, copper accounts for 42 percent, gold 27 percent, followed by silver, aluminum and palladium, each of which accounts for between 8 and 11 percent of the value of the scrap.

Potential for Recycling Precious Metals

While it’s cheaper to mine metals such as copper and iron, the cost of extracting gold, silver and palladium from electronic waste is lower than from commercial mining. Additionally, recovery of these minerals from e-waste reduces the amount that needs to be mined. Several processes are used, but many are environmentally harmful and efforts are being made to find more environmentally friendly methods of precious metal recovery. Three main processes exist.

Using Pyrometallurgy to Extract Precious Metals

The first step in recycling is the dismantling of e-waste along with the removal of easily recycled materials such as plastics, copper wire and metal casings. The remaining waste is crushed and ground into small pieces.

This waste is incinerated or melted in a furnace at high temperature above the melting point of the metals to be recovered. These form a pool of liquid metal, sometimes called collector metal. Using conventional metallurgical processes, copper is removed. while iron and other less valuable metals are lost as slag. Thereafter, gold, silver and palladium waste are recovered. Limitations of this process include the capital cost of the equipment as well as the release of dioxins generated by the remaining plastic and PCB components

Hydrometallurgical Processes

Crushed and ground e-waste is placed in a bath containing a leaching agent that dissolves metal components into an aqueous solution. Depending upon the leaching agent used, it’s possible to select which groups of metals will dissolve. Common leaching agents include cyanide, Aqua Regia, which is a mixture of nitric and hydrochloric acid, thiourea, and thiosulphate. Various techniques are used to recover the precious metals from this solution. Many of the chemicals used are toxic and create environmental risks. Once used, they are not recyclable. Due to the toxicity of the waste, some leaching processes are banned by certain countries.

More recently, it has been discovered that a mild solution of acetic acid mixed with an oxidant can dissolve gold, although it’s not clear if this process is commercially available.

Biological Leaching Processes

Using microorganisms, it’s possible to leach precious metals from e-waste and dissolve them into a solution. This is known as biological leaching. One method for dissolving gold entails using a cyanogenic bacteria, chromobacterium biolaceum, in an alkaline solution to mimic the cyanide process. Other forms of bacteria target copper, zinc, lead and aluminum.

While these processes are still in their infancy, work is continuing to establish the right conditions for optimum recovery and to scale pilot plants for commercial production. Apart from the potential to achieve high precious metal recovery rates, these processes are more sustainable and have limited impact on the environment.

Sustainability

Pyrometallurgical and hydrometallurgical processes to recover precious metals from e-waste are well developed, but relatively expensive. However, they are cheaper than mining precious metals and, as the number of dumped electronic devices increases, will be more widely used due to a need to increase the rate of e-waste recycling.

It’s possible to reduce the impact of e-waste recycling processes through proper plant design and as better techniques are developed and become mainstream. Newer processes offer the potential to reduce the environmental impact of e-waste recycling as well as to lower costs.

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